1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device.
2. Description of the Related Art
In recent years, transistors that are formed using a semiconductor thin film having a thickness of several nanometers to several hundreds of nanometers over a substrate having an insulating surface such as a glass substrate have been attracting attentions. Transistors are widely used for electronic devices such as ICs (integrated circuits) and electro-optical devices. In addition, transistors are used as switching elements of active matrix display devices typified by liquid crystal display devices and active matrix light-emitting devices typified by light-emitting devices including organic electroluminescence (EL) elements.
The ranges of uses of such an active matrix display device and such a light-emitting device are expanding, and demands for larger screen size, higher definition, and higher aperture ratio are increasing. In addition, these devices are required to have high reliability, and production methods of these devices are required to provide high yield and to have low production cost.
A reduction in the number of photolithography processes in manufacture of transistors for an active matrix display device and an active matrix light-emitting device is important for cost reduction. For example, one photomask for the eighth generation glass substrate costs tens of millions of yen, and one photomask for the tenth generation glass substrate or the eleventh generation glass substrate costs hundreds of millions of yen. Moreover, even when only one photolithography step is added in the manufacturing process, the number of steps relating to the photolithography step is significantly increased. Therefore, many techniques for reducing the number of photolithography processes have been developed.
As typical means for reducing the number of photolithography processes in a manufacturing process of a transistor, a technique using a multi-tone mask (also called a half-tone mask or a gray-tone mask) is widely known. As examples of the known technique for reducing the number of manufacturing steps by using a multi-tone mask, Patent Documents 1 to 3 are given.    Patent Document 1: Japanese Published Patent Application No. 2012-178545    Patent Document 2: Japanese Published Patent Application No. 2011-155303    Patent Document 3: Japanese Published Patent Application No. 2009-124124